Chap. 36 BIRDS CONQUEST OF THE AIR 743 



cellaneous feeders have well-developed crops. In pigeons, the lining secretes 

 "pigeon's milk." This is the first food of young pigeons and they reach 

 down their parent's gullet to collect it. An air-filled crop is the pout of the 

 pouter pigeon. Some birds quickly empty their crops when they are frightened 

 into sudden flight. This is a bird's involuntary reaction against extra weight. 

 There are two divisions in the stomach, the first and smaller one (proven- 

 triculus) has thin glandular walls which secrete the gastric fluid. In grain-eating 

 birds — pigeons, chickens, and turkeys, in insect eaters, and some others, the 

 second section of the stomach is a well-developed gizzard. Its walls are com- 

 posed of two great muscles whose tendons are brilliantly iridescent. Its inner 

 layer of cells produces a fluid that hardens into the tough lining that is peeled 

 out when the gizzard is prepared for cooking. Grain-eating birds swallow small 

 stones and gravel that grind against the food, without which their gizzards are 

 useless. The great muscles contract again and again grinding the gravel against 

 the already softened food. Birds such as owls, hawks, gulls, and ducks that eat 

 flesh and plants, have poorly developed gizzards or none. In flying birds, the 

 large intestine is relatively short. It is kept almost clear of waste, another way 

 of decreasing the flight load. 



Circulation of Blood. In birds, the circulation of blood differs from that of 

 reptiles in one very important respect. In most reptiles, the oxygen-rich blood 



eiitceiiuM 



CEREBELLUM^ 

 6PT1C LOBES 



OLrMTOHV 



lose 



URETER 

 OIL CLANO SPERM OUCT 



CAECUM 



OPENIN* OF 

 UKCTCR 

 CLOACA 

 OPCNINS OF 

 SPEKH Due 



Fig. 36.15. Diagram of the general structure of a bird (except the air sacs), the 

 domestic fowl. The crop is a storage pouch formed by an enlargement of the 

 esophagus. It is highly developed in seed eaters and practically absent in fish eaters. 

 The stomach includes two sections, the proventiculus whose walls secrete the di- 

 gestive juices and the heavily muscular gizzard where grinding occurs. (Courtesy, 

 MacDougall and Hegner: Biology. New York, McGraw-Hill Book Co., 1943.) 



